AC/DC converters have been known for many years and are used extensively particularly throughout the electronics industry. These converters take power from a utility lines or mains to supply and deliver a predetermined DC output, typically at the lower voltages, which are generally required by electronic circuitry as used in industrial, communications, computing, medical and general applications. Typically, an AC/DC converter comprises a pre-regulator which performs a “power factor correction,” namely, it draws current largely proportional to the input voltage and then feeds a boosted line voltage, typically at 400V, to a capacitor which stores energy during the low voltage parts of the AC input cycle. This capacitor then feeds a DC/DC converter which provides the required DC voltage for the circuitry in question.
There are, however, problems associated with the known types of AC/DC converters. First of all, many of these converters experience problems with the filtering of electromagnetic interference (EMI) caused by the operation of the converter. Every switching stage of the converter generates interference, and modern power converters operate with switching at a frequency typically in the range of 20 KHz to 1 MHz or higher, and such switching can generate significant interference to other electrical equipment. This interference is gerenally reduced by using more suitable switching techniques and/or filtering. Particular challenges in this area lie in filtering the current ripple in the input boost choke or flyback stage as typically used in power-factor correction stages and also in limiting the common-mode noise generated in the isolation transformer between the line/mains and the electronic circuitry being fed by the supply. Industry-led and regulatory standards apply to EMI performance and safety isolation in power converters and it is important to provide a power converter that adheres to these standards.
Another design problem associated with the known types of AC/DC converters concerns the efficiency of the converters. In particular, there is a design challenge in getting a generally optimum efficiency from power converters across the wide range of input voltages, where the input voltage cycle peak varies from typically 1.414*85V to 1.414*265V depending on the country. Furthermore, one increases efficiency at the different points of the operating cycle between 0V and the peak input voltage value. Design of AC/DC converters with an approximately fixed value of bulk capacitor voltage typically involves compromises in optimizing efficiency across the differing line voltages.
In many instances, AC/DC converters have to be capable of operating in dual mode, namely, be capable of taking both low-line voltages (nominally 110V, as is common in the US and Japan) and high-line voltages (nominally 220V to 240V as is common in Europe). It has been known to implement an AC/DC converter having a dual mode input stage whereby either low-line or high-line input voltages can be handled adequately and converted to provide power to a piece of equipment. However, there are various problems with these types of converters including those outlined above.